Humans Smell Like Bloodhounds

By Bjorn Carey |
August 31, 2005 05:34am ET

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When it
comes to finding the source of a scent, nothing beats a bloodhound. Now a new
study proves that humans can also find the origin of odors.

Researchers
piped scents into each nostril of volunteers and tested whether the subjects
could reliably tell which nostril the scent was coming from. The researchers
also looked at the subjects' brain activity--using functional magnetic resonance
images (fMRI)--to figure out how the brain sorted this out.

What they
found was that smelling works a lot like hearing--at least when trying to
pinpoint the source. The brain is set up to locate smells by interpreting scent
intensity and time of arrival from each nostril, much like how it localizes
sounds by contrasting input from each ear.

"It has
been very controversial whether humans can do egocentric localization, that is,
keep their head motionless and say where the spatial source of an odor is,"
said study coauthor Noam Sobel of UC Berkeley. "It seems that we have this
ability and that, with practice, you could become really good at it."

Previous
studies investigating the method for scent location were flawed because the
scents tested stimulated the trigeminal nerve as well as the olfactory nerve.
The trigeminal nerve is mainly responsible for motor movements and touch
sensation in the face, but can sometimes be stimulated by strong odors.

The
olfactory nerve, however, is responsible solely for smelling. To figure out how
smell localization works, Sobel and his colleagues chose scents--essence of rose
and cloves--that only stimulated the olfactory nerve. They also used two scents--vinegar-smelling
propionic acid and banana-smelling amyl acetate--to test the trigeminal nerve.

They pumped
scents into the noses of 16 subjects using a specially designed mask that
provided separate air flow to each nostril. They also tested five volunteers
with no olfactory nerves, a condition called anosmia.

Each of the
normal subjects could tell which nostril was doing the smelling, but the
anosmic volunteers had no idea. Sobel said this result shows that humans use
their olfactory nerve to locate smells.

Each
patient lay still in the fMRI machine--which shows what parts of the brain are
active--while doing the smell test. Data from the fMRI supported the volunteers'
claims - smells from each nostril activated separate areas of the brain's smell
center, the primary olfactory cortex.

"Together,
these results are the first description of the mammalian brain mechanisms for extracting
spatial information from smell," Sobel said.

For future
experiments, Sobel and his colleagues plan to train subjects to track odors in
the field and determine humans' odor localization limits.

Bjorn Carey is the science information officer at Stanford University. He has written and edited for various news outlets, including Live Science's Life's Little Mysteries, Space.com and Popular Science. When it comes to reporting on and explaining wacky science and weird news, Bjorn is your guy. He currently lives in the San Francisco Bay Area with his beautiful son and wife.